1. Field of the Invention
The present invention relates generally to off-chip buses for integrated circuits, and more specifically to forming a bus via in an integrated circuit for connecting bus conductors to off-chip contact pads on the backside of the die containing the integrated circuit.
2. Description of the Prior Art
A backside bus is a bus within an integrated circuit having conductors connected to contact pads on the backside, or the side opposite that on which active devices are formed, of a substrate. There are a variety of circumstances under which a backside bus is useful in integrated circuitry. Sensor devices, whether for image detection, state switches, or chemical sensors, can particularly benefit from backside buses, which increase the front side surface area of a die available for sensors rather than for contact pads. Pad limited designs, in which the integrated circuit is restricted by the number of contact pads which may be fit along the periphery of a given die, may also benefit significantly from backside buses.
Backside buses may be fabricated by forming through-holes within a substrate for a conductive via connecting a contact on the active side of the substrate with a contact on the backside of the via. A separate through-hole is thus required for each bus conductor, such that the substrate area required for conductor is substantial. Additionally, the challenges of filling a through-hole or otherwise reliably establishing an electrical connection from one end of the through-hole to the other are significant.
It would be desirable, therefore, to provide a technique for implementing backside buses in an integrated circuit with minimal use of substrate area and reliable formation of electrical connections between the active devices on the active side and the contact pads on the backside.